The pipe making process for manufacturing an electric resistance welded steel pipe (hereinafter referred to as “electric resistance welded pipe making process”) includes, for example, as shown in FIG. 3, a series of steps of rounding a steel strip 1 in a width direction by roll forming 2 while continuously feeding the steel strip, joining both ends of the rounded steel strip in the width direction by electric resistance welding 3 to make a steel pipe 4, and performing an outer surface bead cutting 6 of cutting and removing an outer surface bead that is a swelling formed on the pipe outer surface side of a seam portion 5 that is the joined portion. In some cases, in combination with the outer surface bead cutting 6, inner surface bead cutting 7 is performed to cut and remove an inner surface bead that is a swelling formed on the pipe inner surface side of the seam portion 5. The combination of the outer surface bead cutting 6 and the inner surface bead cutting 7 is referred to as the inner and outer surface bead cutting.
When an electric resistance welded steel pipe is subjected to secondary processing, such as plastic working of bending and the like, or welding of its pipe outer surface and an end of a separately prepared plate along the pipe length direction, the electric resistance welded steel pipe is often required to be processed with the seam portion disposed at the optimum position adapted to each processing form. A steel pipe 4 for such secondary processing is desired to have a seam portion that can be identified at a processing site.
In general, identification of the seam portion is performed visually or magnetically. However, these have the following drawbacks.
That is, in the identification by visual observation, because the electric resistance welded portion in the state as formed is darkened compared to the other portion owing to oxidation, this dark portion can be identified as the seam portion. However, by undergoing heat treatment such as tempering after pipe making, the entire surface of the pipe body is darkened, and the identification of the seam portion becomes impossible. Further, an outer surface portion of the seam portion that is subjected to the outer surface bead cutting is depressed compared to the other outer surface portion, and this depression is recognizable even after heat treatment. Therefore, after heat treatment, the depression can be recognized and identified to be the seam portion. However, after the pipe is drawn, the outer peripheral shape of the pipe body is uniformized, and the depression becomes unrecognizable, and the identification of the seam portion after pipe drawing becomes impossible.
Further, in the identification by magnetism, because there is a magnetic characteristic difference due to a difference in metallographic structure between the seam portion in the state as formed and the other portion, the seam portion can be identified by detecting the magnetic characteristic difference. However, by undergoing heat treatment such as tempering after pipe making, the difference in the metallographic structure disappears, therefore the magnetic characteristic difference becomes undetectable, and the identification of the seam portion becomes impossible.
On the other hand, as a conventional technique that makes the seam portion recognizable by other means, there is proposed a method (for convenience, referred to as conventional method) in which, for example, a rolling tool is pressed against a cut surface formed by cutting and removing a bead on the pipe outer surface side of the seam portion, and an uneven processed portion is formed. (see Patent Literature 1).
According to the conventional method, owing to the rolling tool, the uneven processed portion remains without disappearing even after annealing or after pipe drawing. Therefore, using this as a mark, the seam portion can be easily identified at the secondary processing site.